The present invention relates to a hydraulic pressure-control system. More particularly this invention concerns such a system usable for controlling a vehicular automatic transmission.
A standard automotive transmission of the automatic type has various hydraulically actuated clutches and brakes which are operated by means of a switching pressure that changes with the load. In such arrangement it is necessary to vary the switching pressure with the loading of the engine and, as much as possible, of the torque converter so that loading peaks on the transmission and the vehicle during shifting operations are largely eliminated. Such loading peaks put a very large strain on various components of the drive train, and also cause the vehicle to jump or hesitate which is uncomfortable for the riders.
This disadvantage can be largely avoided by creating a control pressure by means of which the switching or shifting pressure is established. The two pressures are ideally steplessly varied. However, due to the necessity of normally using electrical components it is almost invariably necessary to provide a stepped variation of at least the control pressure.
Such an arrangement normally is constituted by a proportional magnet connected to the control valve and fed current in proportion to the pressure it is to exert on the control valve and, therefore, the pressure the control valve is to establish. Since an automotive vehicle generally operates with a 12-volt system, it is necessary to make the magnet respond to relatively small steps of relatively low voltage. Furthermore, it is necessary to reduce friction within the magnet to an absolute minimum.
Thus it is necessary that the control magnet be an extremely precise and carefully constructed element. Any variation from extremely exact tolerances will cause the system to malfunction so that this component must be of the absolute best grade and, therefore, very expensive. Any deviation from within a relatively narrow tolerance range cannot normally be adjusted out. Furthermore, such an extremely precise magnet often has a relatively short service life in a motor vehicle where it is subjected to considerable shock and physical stresses.